The invention relates to a rotational rheometer.
The basic structure of rotational rheometers is known from Austrian patent 404 192. A rotational rheometer of the type referred to above is known from DE-A-34 23 873. This document describes a rotational rheometer whose rotor is coupled to a test specimen and is suspended in a stator by a low-friction bearing system. A compensation arrangement which compensates for bearing torque on the rotor over the full range of rotation of the rotor is provided. Also provided are position transducers which are able to determine accurately the angular position of the rotor over the full 360xc2x0 range of rotation. Also provided is a transducer with which the longitudinal position of the rotor relative to the stator can be accurately detected. The disadvantage is that, with this rheometer, thermal expansion, the rigidity of the stand, and temperature drift in the stand affect the measurement process.
An object of the present invention is to avoid having indirectly to measure the distance between the measuring elements of rotational rheometers having these basic principles, i.e. to avoid having to take the circuitous route of measuring the distance between a point on the measuring shaft and a point on the stand or stator. Instead, the object is to be able directly to measure, and/or set, and/or keep constant the distance between the measuring elements forming the measurement gap S. This is important, since even minor fluctuations in the depth of the measurement gap S in the course of the measurement process, e.g. as a result of fluctuations in temperature, in particular of the stand and/or the measuring elements, have a considerable effect on the accuracy of measurement.
According to the invention, non-contact position sensors are therefore provided, or are supported by one of the two measuring elements bounding the measurement gap S, to establish or measure, and/or set, and/or keep constant the depth of the measurement gap S. The other respective measuring element supports the component which causes the position sensor to respond, or itself causes the position sensor to respond. The output signals of the position sensors are supplied to the analyzing unit; advantageously, the output signals of the analyzing unit, in dependence upon the output signals of the position sensor, control a device for modifying or setting the measurement gap by adjusting the height at least of one of the two measuring elements.
Preferred embodiments of the invention employ different embodiments or variants of position sensors which enable the depth of the measurement gap S to be measured very precisely in a non-contact manner, or which react very sensitively to fluctuations in the distance between the mutually opposite measuring elements.
The accuracy of the position sensors used according to the invention is sufficient to achieve the accuracy required in setting the depth of the measurement gap. The measurement errors hitherto caused by the lack of accuracy in setting the depth of the measurement gap are largely eliminated thereby.
Another aspect of the invention enables measurement errors caused by fluctuations in temperature to be largely eliminated and advantageously enables rapid analysis to take place.
In the analyzing unit, the measurement gap depth values measured with the position sensors are combined with the measurement values for the moment of the substance to be tested and, if need be, with the measurement values of a normal force measurement device; they are then used to calculate viscosity.
In a rotational viscometer where measurements are taken of a specimen or substance with the height h, which is generated by the depth of the measurement gap S between a fixed measuring element (plate) and a measuring element (plate) which rotates relative thereto and has a radius R, the following relationships apply to the shear rate D (1) and viscosity xcex7 (2):                               D                      (            R            )                          =                              ω            *            R                    h                                    (        1        )                                η        =                              τ                          D                              (                R                )                                              =                                                                      2                  *                  M                                                  π                  *                                      R                    3                                                              *                              1                                  D                                      (                    R                    )                                                                        =                                          2                *                M                *                h                                            π                *                                  R                  4                                *                ω                                                                        (        2        )            
If, for example, a constant torque M is pre-selected, a change in the height h causes the angular velocity xcfx89 to fluctuate in the same proportion, as a result of which the calculated viscosity remains constant. If a height fluctuation is not allowed for in the calculation, however, the following error results for the viscosity xcex7.
If hxe2x80x2=k*h (error factor k) is used for the height, the result is equation (3) for the true angular velocity:                               ω          xe2x80x2                =                                            D                              (                R                )                                      *                          h              xe2x80x2                                R                                    (        3        )            
and equation (4) for the viscosity established:                     η        =                                            2              *              M              *              h                                      π              *                              R                3                            *                              D                                  (                  R                  )                                            *                              h                xe2x80x2                                              =                                    C              *                              h                                  h                  xe2x80x2                                                      =                          C              *                              1                k                                                                        (        4        )            
h=calculated specimen height [m]
hxe2x80x2=true specimen height [m]
D(R)=shear rate at the radius xe2x80x9cRxe2x80x9d [1/s]
xcfx89=calculated angular velocity [1/s]
xcfx89xe2x80x2=true angular velocity [1/s]
xcfx84=shear stress [Pa]
M=torque [Nm]
xcex7=viscosity [Paxc2x7s]  C  =                    2        *        M                    π        *                  R          3                *              ⁢          D              (        R        )            
It is clear from the above derivation that, if there is a measurement error in the specimen height, viscosity fluctuates in inverse proportion to the height ratio, that is to say, a +1% height measurement error produces a 1% reduction in viscosity. The measurement gap is generally 1 to 2 mm; for a viscosity error of  less than 1%, the size of the gap therefore has to be determined with an accuracy better than 10 xcexcm or 20 xcexcm.
Advantageous embodiments of the invention will become clear from the following description, claims, and drawings.